The process of semiconductor manufacture entails a precise sequence of alternating microlithographic steps and wafer treatment steps such as etching or ion implantation. Any given microlithographic step forms a pattern of photoresist on the wafer to define the portions of the wafer that are subjected to the subsequent treatment step. The microlithographic steps, of which there are on the order of eight to twelve, are the most critical in the wafer fabrication process since they pattern the device levels prior to subsequent irresversible processing steps. Not only must each photoresist pattern be correct in itself, but each must be precisely registered to the patterns that have been formed in previous steps.
Current fabrication practice is to measure critical dimensions and overlay registration after microlithography steps, since these measurements contain a great deal of information regarding the general status of the process. Critical dimensions outside specified limits indicate problems with projection system focus and exposure or improper development of the photoresist, while mask registration measurements outside specified limits indicate that the most recently projected mask level is not properly aligned with other pattern levels on the wafer.
On the basis of these measurements, the process engineer can diagnose and correct process problems before more wafers are incorrectly patterned. If the wafers are valuable or have been processed to a high level, the most recent photoresist layer can be stripped from the wafer and the microlithography process repeated for that particular mask level.
The increased complexity of VLSI wafer processing equipment demands even more measurement and inspection than previously. The projection systems in use now approach the physical limit of light diffraction for lines below 1.5 microns in width. At the same time, the market needs are such that etching machines are expected and required to produce square 1-micron profiles with 1:1 aspect ratios. Needless to say, small deviations in the operating parameters of these machines can be disastrous.
Current methods of measuring critical dimensions utilize optical microscope image analysis, helium-neon red laser edge scatter, or a scanning electron microscope. The first two may also be used for mask registration measurement. However, as will be discussed below, each method presents certain problems that render the method less than ideal.
The first method, microscope image analysis, utilizes a television camera or a scanning slit attached to the camera port of a white light illuminated microscope. The image or scan signal is transmitted to a digitizer and thence to a dedicated computer. The operator chooses, by means of a cursor or cross hairs, the critical dimension to be measured, and the computer calculates the line width from the dark-to-bright transitions on the digitized image. The system is normally calibrated to a standard photo mask.
While optical microscope image analysis has been used widely, and is popular because of its relatively low cost and ease of operation, as the scale of semiconductor devices has shrunk, the method has been unable to provide the necessary performance. Thus, lack of resolution, diffraction from line edges, interference from underlying structures and layers, and an inability to determine shapes of line edges render this method unsuitable for measuring sizes or shapes of lines smaller than approximately 2.0 microns.
The second method employs a scanning red laser, the beam of which is focused by an objective lens to a microspot of about 1 micron in diameter. The microspot is focused on the surface of a wafer which is moved under the microspot, and suitably placed detectors detect scattered light. Where there are no structures on the surface, a very low level of scattered light is detected; at an edge, light is scattered by the spot-edge interaction.
Line widths are defined by the distance between the detected edges, and while this measurement method appears to work well on photo masks, it works poorly on wafers. This is because the red laser light used for edge detection is transmitted by most wafer processing layers such as photoresist, polysilicon, and silicon dioxide. The light transmitted through transparent layers causes interference effects which degrade measurement.
The third method of measuring critical dimensions (but not mask registration) utilizes a scanning electron microscope (SEM). SEM's are capable of magnifying an image on a wafer more than 50,000 times and therefore are useful for detailed wafer inspection.
However, a number of problems render the SEM unsuitable for use as an in-process tool. One class of problems arises from the fact that SEM's capable of measuring wafer structures nondestructively must operate at a very high vacuum (10.sup.-7 torr). Inherent problems with cycling high-vacuum systems limit the throughput to approximately ten wafers per hour at five measurements per wafer. High precision stages must also be developed which can cycle continuously in high vacuum with no lubrication. A second class of problems results from the basic physics of SEM operation. Since the magnification is read from currents flowing through scan coils, since the electron trajectory is affected by the inevitable column contamination, and since the nature of the secondary electron signal is to provide very low contrast, SEM's are characterized by poor reproducibility. A third class of problems relates to longevity. Presently available electron sources such as lanthanum hexaboride last no more than about 500 hours.
Moreover, since SEM's can only view surface structure, they cannot be used for registration measurement, and therefore cannot be the basis for a combined critical dimension and registration instrument.
Thus, it can be seen that the prior art systems cannot make measurements at the small scale now required, cannot provide the high throughput, reproducibility, and reliability necessary for in-process measurements, or cannot provide the full range of measurements needed. What is needed is an instrument capable of accurately measuring features in the 0.5-1.5 micron range (as well as larger features) at a high throughput (say 60 wafers per hour) while providing the capability of determining registration and alignment from one layer to the next.